1. Field of the Invention
The present invention relates to a method of producing an optical waveguide, in particular, a flexible polymer optical waveguide.
2. Description of the Related Art
The methods of producing a polymer optical waveguide which have been proposed include (1) a method in which a film is impregnated with a monomer followed by selectively exposing a core portion to vary the refractive index further followed by laminating a film (selective polymerization method), (2) a method in which a core layer and a cladding layer are coated followed by forming a clad portion by use of a reactive ion etching (RIE method), (3) a method in which a photolithography method in which by use of a UV curable resin that is obtained by adding a photosensitive material to polymer, exposure and development are performed is used (direct exposure method), (4) a method that makes use of an injection molding, (5) a method in which a core layer and a cladding layer are coated followed by exposing a core portion to vary the refractive index of the core portion (photo-bleaching method) and so on.
However, the selective polymerization method according to (1) has a problem in lamination of films; methods according to (2) and (3), owing to the use of the photolithography, result in high cost; and the method according to (4) has a problem in accuracy of an obtained core diameter. Furthermore, the method according to (5) has a problem in that sufficient difference of the refractive indexes cannot be obtained between the core layer and the cladding layer.
At present, practical methods excellent in the performance are only those according to (2) and (3); however, these have the problem of cost as mentioned above. Furthermore, all of methods (1) through (5) cannot be applied to form a polymer optical waveguide on a large area and flexible plastic material.
Furthermore, as a method of producing a polymer optical waveguide, a method is known in which a patterned substrate (clad) where a groove pattern that becomes a capillary is formed is filled by a polymer precursor material for a core followed by curing to form a core further followed by laminating a plane substrate (clad) thereon. However, in the method, not only in the capillary groove but also totally between the patterned substrate and the plane substrate, the polymer precursor material is thinly filled and cured to form a thin layer having the composition the same as the core layer; as a result, there is a problem in that light leaks through the thin layer.
As a method to overcome the problem, David Heard has proposed a method in which a patterned substrate on which a pattern of a groove that becomes a capillary is formed and a plane substrate are solidly fixed by a clamping jig and furthermore a contact portion of the patterned substrate and the plane substrate is sealed with a resin followed by reducing the pressure to fill a monomer (diallyl isophthalate) solution into the capillary, and thereby a polymer optical waveguide is manufactured (Japanese Patent No. 3151364). This is a method in which as a core forming resin material, instead of the polymer precursor material, a monomer is used to make the viscosity of a filling material lower, and thereby the monomer is allowed to fill the capillary by making use of a capillary phenomenon and not allowed to fill other than the capillary.
However, since the method uses a monomer as the core forming material, when the monomers are polymerized to form a polymer, the volume contraction rate becomes large; as a result, there is a problem in that a large transmission loss of the polymer optical waveguide is caused.
Furthermore, the method is a troublesome method in which the patterned substrate and the plane substrate are firmly held by use of the clamp or additionally the contact portion is sealed with a resin, and thus the method is not suitable for the mass-production. Accordingly the cost reduction cannot be achieved. Still furthermore, it is impossible to apply this method to manufacturing a polymer optical waveguide when a film having a thickness of millimeter order or 1 mm or less is used as the cladding material.
Still furthermore, recently, George M. Whitesides et al. of Harvard University, as a novel technology to obtain a nano structure, have proposed a method called a capillary micro-mold as one of soft lithography. This is a method in which a master substrate is formed by making use of photolithography; by making use of adhesiveness and peelability of poly-dimethylsiloxane (PDMS), a nano structure of the master substrate is replicated on a template of PDMS; and liquid polymer is filled in the template by making use of the capillary phenomenon followed by solidifying it. In Scientific American, September 2001 (Nikkei Science, December 2001), a detailed explanatory article can be found.
Furthermore, a patent as to the capillary micro-mold method has been applied by Kim Enoch et al of a group of George M. Whitesides of Harvard University (U.S. Pat. No. 6,355,198).
However, even when the producing method described in the patent is applied to the manufacture of a polymer optical waveguide, since a cross section area of a core portion of the optical waveguide is small, the formation of the core portion takes a long time. Accordingly, the method is not suitable for the mass-production thereof. Furthermore, there is a disadvantage in that when a monomer solution polymerizes to form polymers, a volume change is caused to result in change in shape of the core; and thus the transmission loss becomes larger.
Furthermore, B. Michel et al. of IBM's Zurich Research Laboratory have proposed a high resolution power lithography technique that uses PDMS and reported that according to the technique the resolution power of several tens nanometers could be obtained. A detailed explanatory article is described in IBM J. REV. & DEV., VOL. 45 No. 5 Sep. 2001.
Thus, the soft lithography technique and capillary micro-molding method that use PDMS are recently gathering attention as nano-technology primarily in USA.
However, when an optical waveguide is prepared by use of the micro-molding method as mentioned above, making the volume contraction rate smaller during the curing (that is, making the transmission loss smaller) and lowering the viscosity of a filling liquid (monomer or the like) to realize easy filling cannot be rendered compatible. Accordingly, when the transmission loss is preferentially made smaller, since the viscosity of the filling liquid cannot be reduced to a certain limit or less, the filling speed becomes slower, and the mass-production cannot be expected. Furthermore, the micro-molding method presumes the use of a glass or silicon substrate as the substrate, that is, a flexible film material is not considered to use.
It is desired from a viewpoint of expanding applications to expand the degree of freedom of selection of materials that can be used as the cladding film substrate. Furthermore, in the method of producing a polymer optical waveguide, as the core forming curable resin and the cladding layer forming curable resin, furthermore as an adhesive for adhering a cladding layer (cladding film) to a core formation surface, a UV curable resin, a heat-curable resin or the like are used; however, the UV curable resins and the heat-curable resins, depending on the kind thereof, are not necessarily sufficient in the adherence with the cladding film substrate. Accordingly, an improvement in adhesiveness of the cladding film substrate and a core and a cladding layer is also desired.
On the other hand, as the surface modification method of polymers, various methods have been known. Recently, JP-A Nos. 2002-365429 and 2002-146066 have proposed a method in which short wavelength UV light is irradiated on a surface of polymer to heighten the hydrophilicity thereof. JP-A No. 2002-365429 discloses a method in which UV light having a wavelength of 200 nm or less is irradiated on an adhesion surface of a transparent protective film for use in polarization plate that is adhered to a polarizer through an adhesive layer to increase the hydrophilicity of a surface of a protective film, and thereby improving an anchor effect with the polarizer that is hydrophilic. The use of a low-pressure mercury lamp and a xenon excimer lamp are shown as the UV treatment with a UV light source having a wavelength of 200 nm or less. Furthermore, JP-A No. 2002-146066 describes a method in which as a surface modification method (making hydrophilic) of a fluorinated polymer product, in the presence of water or water vapor, a vacuum UV fluorine laser having a wavelength of 157 nm is irradiated.
However, in JP-A Nos. 2002-365429 and 2002-146066, there are no suggestions of modifying a cladding film substrate and a template surface to improve the filling speed in the micro-molding method.